vifs not working?

Okay, after a hand from two helpful listmembers, I've managed to get
xen to start. dom0 seems to work just fine. But there's something
wrong when trying to run domU machines: I can't make the network work.
The only way I've ever gotten a domU machine to start is to configure
it with no network interfaces at all. When trying to start one with a
network interface, or when trying to xm network-attach an interface to
a machine started with none, I get a long hang (three minutes?) and
then a complaint like (this quoted from an xm network-attach; attempts
to xm create with a nonempty vif list word it slightly differently):
Error: Device 0 (vif) could not be connected. Hotplug scripts not working.
This despite specifying "script=/usr/bin/true" on the network-attach
command line.
What are these hotplug scripts and how can I make them start working?
Or, alternatively, how can I suppress the error? If I start a
networkless machine and use network-attach on it, the xennet* and xvif*
interfaces appear, and, as far as I can tell, they work fine - until xm
errors out and prints a whine like the above, at which point the
xennet* and xvif* interfaces vanish again.
The networkless domU config file I'm using is very simple:
kernel = "/boot/domU"
memory = 128
name = "d1"
#vif = [ 'mac=02:00:00:00:00:01, bridge=null' ]
vif = [ ]
disk = [ 'file:/disks/1,1,w' ]
root = "xbd0"
extra = "-s"
/disks/1 is a 4G file which I set up with the help of a vnd. (When I
try to start the domain, the vnd is, of course, unconfigured.)
Appended after my signature below is dmesg from the domU machine (taken
after an attempt to use xm network-attach on it). I got it off the
domU machine by putting it in a file and then, after shutting down the
domU, using vnd to get the file out of the filesystem.
Any ideas what I'm doing wrong? I can, of course, provide more
information if there's something important I've left out; I'm almost
totally unfamiliar with this stuff, so please consider pilot error a
plausible explanation - just tell the pilot what the error is!
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